IE44193B1 - Active carbon - Google Patents

Abstract

An improved process is disclosed whereby coal, coke or a combination thereof can be converted in good yield to a very high surface area active carbon with a unique and useful combination of properties, which carbons are particularly useful for water purification. Novel active carbons having a very high surface area which substantially have a cagelike structure exhibiting microporosity are disclosed.

Description

This invention relates to a process of producing an active carbon from carbonaceous material and the active carbon product produced thereby and, more particularly, to a staged temperature process for improving the yield and processability during manufacture of a novel, very high surface area, active carbon having a substantially cagelike structure exhibiting microporosity, which carbon is of good bulk density and superior Total Organic Carbon Index.

In accordance with the present invention, a feed of preferably crushed coal, coal coke or petroleum coke, or a mixture thereof, is heated with agitation in the presence of a generally substantial weight ratio of hydrous potassium hydroxide at a first lower temperature to dehydrate the combination, whereafter the temperature is raised to a second higher temperature to activate the combination and the resulting product is cooled and' generally washed to remove inorganic components therefrom so as to form a very high surface area, active carbon having a substantially cage-like structure exhibiting microporosity, which carbon is of good bulk density and superior Total Organic Carbon Index.

In U.S. Patent No. 3,624,004, a process is disclosed in which a pyrolysate, which is made by heating above the salt decomposition temperature an aromatic acid and at least enough of an electrolyte, e.g., potassium 419 3 hydroxide, to form a salt of the aromatic acid, is subjected to controlled oxidative activation above 1500°F. in the presence of carbon dioxide to produce a high surface area, low bulk density active carbon.

Xn U.S. Patent No. 3,642,657, processes are disclosed in which low surface area, active carbons are made by decarboxylation of petroleum coke acids and such decarhoxylated materials are further heated above about 1100°F. with a solute, for example potassium hydroxide, in the presence of a hydrogen halide, carbon monoxide or carbon dioxide, to form a high surface area, low hulk density, active carbon.

In U.S. Patent No. 3,817,874, a process is set out for increasing the surface area of a low to intermediate surface area, active carbon by heating such carbon above about 1100°F. with sodium or potassium hydroxide in the presence of carbon dioxide. The process produces high surface area, active carbons oi moderate bulk density.

In U.S. Patent No. 3,833,514, a proeess is set forth to produce a high surface area, active carbon by heating the salt of an aromatic acid admixed with an electrolyte, e.g. potassium hydroxide, above the decomposition temperature of the salt. The process produces active carbons of low bulk density. 441® 3 Although the active carbons produced in the above processes are of high effective surface area as measured hy BET and of generally good properties, several process and product deficiencies are noteworthy: (l) the processes are of less than maximum yield based upon carbonaceous feed consumed when the more economical hydrous alkali is used, (2) when coke acid carbonaceous feeds are used, the total process requires extra processing steps, and (3) the feed combination of alkali and carbonaceous material during calcination forms a sticky, viscous mass which is very difficult to handle in commercial operations because of adhesion to the walls and plugging of the calcination (activation) vessel. The source of problems (l) and (3) when potassium hydroxide is employed appears to lie in the use of the more economical hydrous alkali for continuous processes wherein spent carbon wash solutions are evaporated and treated to recover alkali for recycle purposes. More particularly, yield loss is thought to be due to oxidative attack by water vapour during high temperature calcination of the alkali-carbonaceous feed combination.

Furthermore, the bulk density of the active carbon made from an aromatic acid or petroleum coke acid is lower than is desirable for many commercial applications and the Total Organic Carbon Index per volume of carbon used, an important and industry-recognised measure of the power of the active carbon to remove organics from water effluents, is far from ft 4419 3 maximized. Also, the effective BET surface area of such carbons while high is still not maximized.

Now, a higher yield process has been developed which uses hydrous potassium hydroxide and substantially eliminates the processing problems referred to above. Further, the process can produce a carbon with the unique combination of good bulk density, very high surface area and excellent Total Organic Carbon Index.

The key to the new process is to carry out the heating of the hydrous alkalicarbonaceous feed combination in two steps wherein the first, lower temperature step is carried out with agitation and dehydrates the feed charge prior to the second, higher temperature activation step. The key to the new combination of product properties is the production of a carbon structure in which a substantially larger proportion of the surface is substantially cage-like, which cage-like structure exhibits properties of microporosity.

The present invention provides active carbon having a cage-like structure exhibiting microporosity which contributes to over sixty percent of its surface and which has an effective BET surface area greater than twenty-three hundred square metres per gram and a bulk density greater then twenty-five hundredths of a gram per cubic centimeter.

The invention also provides a process for making active carbon comprising: (a) substantially dehydrating (as hereinafter defined) an agitated combination of solid hydrous potassium hydroxide and a carbonaceous substance which is coal, coal coke, petroleum coke or a mixture thereof by heating said combination below 900°F.; (b) activating the product of step (a) by heating within the range of from 1300°F. to 1800°F., inclusive; and (c) cooling the product of step (b) and removing essentially all the inorganic impurities therefrom to form a high surface area, active carbon.

Reference is now made to the accompanying drawings, in which: Figure 1 shows a phase contrast, electron microscope photomicrograph of a carbon of this invention under a low total magnification (X 142,000), and Figure 2 shows a phase contrast, electron microscope photomicrograph of a carbon of this invention under high total magnification (X 3,116,000). 41-93 The carbonaceous feeds foi’ the instant invention are coal, coal coke, petroleum coke or mixtures thereof. Such feeds are preferably utilized in a pulverized form, preferably less than about tenmesh and, g more preferably, below about twenty mesh (mesh sizes are U.S. standard mesh) The carbonaceous feeds generally contain from about one to ^bout ten percent of sulfur and from about three to ahout twenty percent of volatiles. Preferably, the volatiles and. sulfur content are on the low side of the above figures to maximize the product yield, and to improve the efficiency of the alkali recycle hut this is not critical and depends upon the feeds available.

The process according to the invention is generally carried' out as follows. Prior to processing, the carbonaceous feed is intimately mixed with solid hydrous potassium hydroxide,generally containing more than two percent by weight of water, preferably in powder or flake form. The lower limit of water content in the potassium hydroxide is set by the economics of removing water from the alkali prior to use, and the upper limit of about twenty-five weight percent is set by the ease of handling the alkali and the alkali-carbonaceous feed mixture during the precalcination step. More preferably, hydrous potassium hydroxide containing from about five to about fifteen weight percent of water is used. The mesh size of the potassium hydroxide is not critical, but the alkali should disperse well with the particles of carbonaceous feed. 41« 3 Preferably, the potassium hydroxide to carbonaceous feed ratio used is from about one-half to about five parts by weight, more preferably, from about two to about four parts by weight per part by weight of carbonaceous feed. It is particularly preferred that the potassium hydroxide to carbonaceous feed ratio should be from about two and one-half to about three and one-half parts by weight of potassium hydroxide per part by weight of carbonaceous feed.

The carbonaceous feed-potassium hydroxide combination is heated with agitation in the precalcination step in, preferably, an indirectly fired, rotary tube calciner equipped with a rotating auger within a temperature range of from about 600°C to about 900oF., inclusive, and, more preferably, from about 7θΟ°Ρ· to about 75θ°ϊ'., inclusive, preferably for times of from about fifteen minutes to two hours and, more preferably, for about one-half to one-andone-half hours. The upper time limit is not critical and is generally set by economic considerations. What is desired in the preealcination step is a sufficient heating time substantially to dehydrate the combination of feed and alkali and to provide for a substantially uniform reaction. By substantial dehydration is meant producing a solid product from the precalcination step containing not more than two weight percent of water.

In general, at start-up, the preealciner may contain an inert gas blanket of nitrogen, argon, etc., but, once the operation is under way, the gases present during precalcination are generally sufficient to maintain the inertness of the atmosphere necessary for maximum yield.

The product of the precalcination step is then fed generally without cooling or grinding, although either or both of these additional operations may be advantageous, into a second, indirectly fired calciner heated to a temperature of from 1,3OO°F. to l,800°F., and, more preferably, from l,400°F. to l,700°F., for from twenty minutes to four hours, more preferably, from thirty minutes to two hours. The upper time limit is not critical but too lengthy a residence time in the calciner can decrease the yield of active carbon product. Preferably, an indirectly fired rotary calciner Is used as agitation of the calciner mix is beneficial to operation of the process. It is important for good results in the calcination (activation) step to avoid fusion of the material contained in the calciner, In general, the atmosphere in the rotary calciner should be inert’ for maximum, yield and the gases present during reaction are sufficiently inert reasonably to accomplish the preferred Conditions in the closed system preferred for use.

Thereafter, tlie calciner product, particularly if a high sulfur carbonaceous feed is used, is optimally desulfurized; for example, by the steam method set out in U.S. Patent No. 3,726,808.. The resulting .product is then cooled, washed with water to remove inorganic impurities and dried. In a continuous process, it is desirable to reclaim and to recycle the alkali by recausticization and evaporation of the spent wash liquid to form feed alkali.

The active carbon of the instant invention has a cage-like structure which contributes to over sixty percent of its surface and, more preferably, over eighty percent of its surfape and, most preferably, over ninety percent of the carbon surface as measured by phase contrast, high resolution microscopy. This cage-like structure has the feature that the individual cages are of such a size that they exhibit properties of microporosity, i.e., essentially complete filling of the individual cages by the adsorbate at low effective concentration to give a large micropore volume. They are also substantially homogeneous in size (Figure l) as can be seen by low magnification image photomicrographs (e.g., X 140,000) taken by phase contrast, high resolution electron microscopy on a JEOL 100C electron microscope supplied by JEOL Ltd., Colindale, London, England. Using the same type of unit at high magnification (e.g.,X 3,000,000) the individual cages are clearly in evidence and appear to he formed from single sheets of graphitic-type lamellae (Figure 2). This cagelike structure is responsible for the multi-layer adsorption demonstrated by the carbons of this inventim and the extremely large effective surface areas as measured by the BET method.

The active carbon product formed preferably has an effective BET surface area greater than 2,300 square meters per gram and, more preferably, greater than 2,700 square meters per gram and, most preferably, above 3,000 square meters per gram. The active carbon preferably has a bulk density greater than about twentyfive hundredths of a gram per cubic centimeter and, more preferably , greater than twenty-seven hundredths of a gram per cubic centimeter and, most preferably, above three-tenths of a gram per cubic centimeter. Further, the product preferably has a Total Organic Carbon Index greater than 300, more preferably, greater than 500 and, most preferably, greater than 700.

Typical property ranges of active carbon products which can be produced by the process of this invention are shown in the Table below. 419 3 TABLE Property Starting Coke Material Coal Effective Surface Area, BET square meters/gram 3,000-4,000 1,800-3,00(1 Iodine No. 2,200-3,400 1,800-2,00( Methylene Blue Adsorption, mi11i grams/gram 300-600 400-500 Phenol No. 8-16 12-20 Molasses No. 30-70 100-150 Total Organic Carbon Index (TOCI) 300-1,000 300-1,000 Major Pore Radius Range,angstroms* 15-60 ___***¥- Average Pore Radius, angstroms* 20-30 — Bulk Density, grams/cubic centimeter 0.25-0.4 0.24-0.4 Ash, weight percent 1.5 2.4 Water Solubles, weight percent 1 1 pH 7.5-8.5 7.5-8.5 Adsorption Capacity (COg at 195°K.;cc/gram)** 0.9-2 — Cage dimensions, angstroms*** 10-50 --- Measured by nitrogen adsorption using a Digasorb 2500 unit made by Micromerities, Norcross, Georgia ** Assumed density of COg is 1 gram/cc.

*** From image photomicrographs using a JEOL 100C electron microscope **** jjot measured.

The active carbons of this invention are useful for all the uses that prior art active carbons have been used for including water treatment, gas and vapour adsorption, for example, carbon dioxide, methane, nitrous oxide, etc., decolorisation, white wall tire compounding, etc. The carbons of this invention are regeneratable by various methods useful for carbon regeneration purposes.

While the invention is described in connection with the specific examples below, it is to be understood that these are for illustrative purposes only. Many alternatives, modifications and variations will be apparent to those skilled in the art in the light of the examples below, and such alternatives, modifications and variations fall within the scope of the appended claim’s.

General experimental procedure The hydrous potassium hydroxide used contained approximately ten percent by weight water.

Phase contrast, high resolution electron microscopy was accomplished using a JEOL 100C electron microscope supplied by JEOL Ltd., Colindale, London, England.

Determination of effective BET surface area Surface area measurements were accomplished employing the one point BET method using a ten percent nitrogen-ninety percent helium mixture. The active carbon sample was pre12 treated at ambient temperature l'or about one hour in a slow stream of the above gas mixture, then cooled to liquid nitrogen temperature for about forty-five minutes for nitrogen adsorption, and finally warmed to ambient temperature and the composition of the desorbed gas measured with a thermal conductivity detector.

Determination of Total Organic Carbon Index (TOCl) Five separate samples of the active carbon to be determined are prepared having weights of 0.025 g., 0.05 g., 0.10 g., 0.15 g., and 0.25 g., respectively. Each sample is then stirred for one hour in contact with a 500 ml. aliquot of effluent (primary sewage effluents from Chicago Metropolitan Sanitary District and the City of Naperville, Illinois), after which the carbon is removed, by filtration and the filtrate is analysed for total organic carbon (TOC) by the method set out in ASTM D 2579-7¾ to obtain the residual TOC.

The residual TOC of each of the filtrates is then subtracted from the TOC (similarly determined) of a 500 ml. portion of untreated effluent (Οθ) and divided by the weight of the active carbon used to treat the particulate aliquot of filtrate to give TOC absorbed per gram of carbon (PPM/g.) values. The residual TOC values are then plotted versus the PPM/g. values on a log-log scale and the resulting curve is extrapolated to a value of residual TOC equal to Co, and the corresponding value of the PPM/g. is determined. This value, 44ΐθ3 termed, the TOG isotherm value, is then divided by the TOC isotherm value for a standard active carbon (Aqua Nuchar purchased from Westvaco Inc., Covington, Virginia) and multiplied by 100. The resulting number is the TOCI value (see H.J. Forwalt and R.A. Hutchins, Chemical Engineering, April 11, 1966 edition).

Determination of bulk density Two grams of active carbon product were added to a twenty-five milliliter graduated cylinder and the bottom of the cylinder was gently tapped against a wooden surface for ten minutes. The time was found adequate to give no further change in volume of the carbon.

Determination of the Phenol Number The phenol number· was determined by the method set out in A.W.W.A. 3600-66,4.7.2.

Determination of Molasses Number Index (MNX) The molasses number index was determined by measuring the molasses number of filtered corn syrup molasses of a Darco S-51 (Darco is a Trade Mark) reference sample supplied hy ICI America, N.Y., N.Y., and assigning to it a value of 100. The molasses number index is then the molasses number of a carbon of this invention compared with the molasses number of Darco S-51 which is assumed to be 100. To determine the molasses number, a plot of grams adsorbed per gram of carbon versus percent adsorbed was made based upon tests using several weights of carbon and the same total amount of adsorbate, and the molasses number was determined as grams adsorbed per gram of carbon at ninety percent reduction in the original solution concentration of adsorbate.

The invention is further illustrated by the following examples , in which mesh sizes are U.S. standard mesh. Example 1.

Sugar Creek, Missouri, refinery petroleum coke containing about 9.7 percent volatiles and 4.9 percent sulfur, crushed to a mesh size of about forty, was utilized in this Example. A 3/1 KOH to coke ratio was used. The rotary precalciner equipped with a counter rotating auger was held at about 725°F. and the average residence time in the precalciner was about one hour. The rotary calciner was held at about 1550°!'’. and the average residence time in the calciner was about two hours. The cooled product was washed with water and dried at ilO°C. in a vacuum oven. Properties of the active carbon product after washing out inorganics and drying are given in the Table below.

TABLE Yield Bulk Density Effective Surface Area, BET Total Organic Carbon Index Methylene Blue Adsorption Phenol No. 60-65 percent 0.3 grams/cubic centimeter 3,600-3,900 square meters/gram 400-800 500-550 milligrams/gram 10-12 Comparative Example II The crushed, petroleum coke used in this Example was the same as that employed in Example 1. A 3/1 KOH to coke ratio was used. The calciner was held at about 1400°F. (a higher temperature could not be used because of plugging and sticking problems), and the average heating time of the several batch runs comprising this Example varied between 24 and 48 minutes. The cooled product was washed with water and dried at 110°C. in a vacuum oven. The properties of the active carbon product are shown in the Table below.

TABLE 55-58 percent 0.375 gram/cubic centimeter 3,900 square meters/gram 170 2,900 12.5 392 milligrams/gram Yield Bulk Density Effective Surface Area,BET Total Organic Carbon Index Iodine Number Phenol Number Molasses Number Index Methylene Blue Adsorption Example 111 This Example shows the effect of residence time in the calciner upon certain carbon product properties. The precalcination was effected at about 725°F. with a residence time of about one hour with a petroleum coke feed of mesh size about 60 to 100. A 3/1 KOH to coke ratio was employed. The cooled product was washed with water and dried at 110°C. in a vacuum oven. The results are given in the Table below.

TABLE nt a calciner temperature of 1450 °F. Calciner Residence Time (minutes) Effective BET Surface Area (m2/gram) TOCI Bu 1 k Densi ty (grams/cc ) Yield -.(£>- 10 3482 110 0.37 61.5 30 3733 235-750 0.35 59.5 60 3350 430-550 0.34 59.0 120 3736 570-1240 0.35 58.6 at a calciner temperature of 155O°F. 10 3996 910 0.32 58.6 30 4057 910 0.32 58.6 60 3821 1620 0.33 57.7 120 3748 760 Ο.32 57.7 Example IV Illinois No. 6 Coal was crushed to a mesh size between 60 and 100 and precalcined in a 3/1 KOH to coal mixture. The precalciner temperature was 725°F. and a precalciner residence time oi about one hour was used.

The calciner temperature was about 175O°F. with a holding time oi two hours. The cooled product was washed with water and dried at 110°C. in a vacuum oven.

TABLE Yield Bulk Density Effective Surface Area,BET Total Organic Carbon Index Molasses No, Index. 110-120 48-52 percent 0.24 grams/cubic centimeter 2,580 square meters/gram 550 J. 7 419 3 ίο Example V Illinois No, 6 Coal was crushed to a mesh size between 60 and 100, mixed with Sugar Creek Refinery petroleum coke containing about 10 to 12 percent-volatiles and precaleined between 700°F. and 900°F. in a 3/0.25/0.75 KOH/Coal/Coke weight ratio for one hour. The calciner holding time was two hours and the calciner temperature varied as recorded in the Table below. Some properties of the active carbons produced are also set forth in the Table. .15 TABLE Calciner Temperature LU Effective BET Surface Area (m2/gram) Bulk Density (grams/ml) Molasses Number Index TOC I Iodine No. 1,450 3,753 0.315 32 1,300 1,550 2,745 0.283 79 500 2,776 1,550 3,586 0.315 39 320 1,650 3,658 . 0.257 165 750 Example VI Sugar Creek, , Missouri, refinery petroleum coke containing about 10 percent volatiles and five percent sulfur was crushed to a mesh size of forty, mixed with a 3/1 weight ratio of KOH to eolce and precaleined at 725°F. for 0.7 hour. It was thereafter calcined at 1,55O°F. for 1-1/2 hours. The cooled product was washed with water and dried. Properties of the product are set forth in the Table 419 3 below. Photomicrographs of the material are shown in the accompanying Drawings.

TABLE Effective Surface Area, BET 3,704 square meters/gram Iodine Number Methylene Blue Adsorption Bulk Density Phenol Number Total Organic Carbon Index Molasses Number Index 3,226 550 milligrams/gram 0.313 grams/cubic centimeter 9.7 1,500 319

Claims (13)

1. CLAIMS:1. A process for making active carbon comprising: (a) substantially dehydrating (as hereinbefore defined) an agitated combination of solid hydrous 5 potassium hydroxide and a carbonaceous substance which is coal, coal coke, petroleum coke or a mixture thereof by heating said combination below 900°F.; (b) activating the product of step (a) by 10 heating within the range of from 1300°F. to 1800°F., inclusive; and (c) cooling the product of step (b) and •removing essentially all the inorganic impurities therefrom to form a high 15 surface area, active carbon.

2. A process as claimed in Claim 1, wherein said carbonaceous material is coal.

3. A process as claimed in Claim 1, wherein said carbonaceous material is coal coke. 20

4. A process as claimed in Claim 1, wherein said carbonaceous material is petroleum coke.

5. Active carbon having a cage-like structure exhibiting microporosity which contributes to over sixty percent of its surface and which has an effective BET surface area greater than twentythree hundred square meters per gram and a bulk density greater 5 than twenty-five hundredths of a gram per cubic centimeter.

6. Active carbon having a cage-like structure exhibiting microporosity which contributes to over eighty percent of its surface and which has an effective BET surface area greater than twentyseven hundred square meters per gram and a bulk density greater 10 than twenty-five hundredths of a gram per cubic centimeter.

7. Active carbon having a cage-like structure exhibiting microporosity which contributes to over ninety percent of its surface and which has an effective BET surface area greater than three thousand square meters per gran and a bulk density greater than twenty-five hund15 redths of a gram per cubic centimeter,

8. Active carbon as claimed in Claim 5, having a Total Organic Carbon Index (measured by the method defined herein) greater than three hundred.

9. Active carbon as claimed in Claim 8, with an effective BET 20 surface area greater than twenty-seven hundred square meters per gram, a bulk density greater than twenty-seven hundredths of a gram per cubic centimeter and Total Organic Carbon Index greater than five hundred.

10. Active carbon as claimed in Claim 8, with an effective BET 25 surface area greater than three thousand square meters per gram, a bulk density greater than three-tenths of a gram per cubic centimeter and a Total Organic Carbon Index greater than seven hundred. 4419 3

11. A process for making active carbon as claimed in Claim 1, and substantially as herein described with reference to the accompanying drawings.

12. Active carbon when made by a process as claimed in any of 5 Claims 1 to 4, or in Claim 11.

13. Active carbon as claimed in Claim 5, substantially as herein described with reference to the accompanying drawings.